Ukraine crisis: Dozens of people killed in building fire, more shot dead in Odessa

Dozens of people were killed in a fire and others were shot dead when fighting between pro and anti-Russian groups broke out on the streets of Odessa on Ukraine's Black Sea coast, opening a new front in a conflict that has split the country.

In the country's east, pro-Russian separatists brought down two Ukrainian military helicopters involved in an operation to try to dislodge the militants from their strongholds in the town of Slaviansk.

The separatists said three of their number had been killed, along with two civilians, while the defence ministry said two crew from the downed helicopters died and two other servicemen were killed.

"Heavy fighting is continuing," the Ukrainian defence ministry said.

Eastern Ukraine has been a focus of concern in Western capitals since Moscow annexed mainly Russian Crimea from Ukraine in March.

Clashes had also broken out in largely Russian-speaking Odessa, not far from Crimea in the south, but no-one had died until now.

Police said three people were shot dead and dozens of others wounded in running battles between people backing Kiev and pro-Russian activists in the town.

Another 31 people died when a trade union building was set on fire as the fighting continued into the evening.

The total death toll in Odessa later reached 43, Interfax-Ukraine reported.

The United States has condemned "unacceptable" violence on the bloodiest day since Kiev's Western-backed government took power, urging both Ukraine and Russia to restore order.

"The violence and mayhem that led to so many senseless deaths and injuries is unacceptable," State Department deputy spokeswoman Marie Harf said in a statement.

The United Nations Security Council held its 13th emergency meeting on the growing crisis at the second request of Russia in a week.

The Russian ambassador to the UN, Vitaly Churkin, said it is time for the the West to think twice about its reasons for mentoring Ukraine.

"This is not true that Ukraine's government and Ukrainian authorities do not keep the situation under control," he said.

"The situation remains under control but the security situation is threatened by Russian special agents and that's why we are requesting and demanding Russian authorities to stop their efforts that can undermine the regional security and stability."

Ukraine fired on civilians: Putin

Russian president Vladimir Putin's spokesman said Ukrainian forces had fired on civilians from the air in Slaviansk in a "punitive operation" that destroyed an international peace plan.

Moscow has tens of thousands of troops massed on the border and claims the right to invade if needed to protect Russian speakers.

The Western-backed government in Kiev said the use of missiles to bring down its helicopters showed Russian forces were in the town. Moscow denies its troops are on the ground.

Ukraine's acting president, Oleksander Turchinov, also said Russian "armed saboteurs" had tried to enter the country overnight, but were pushed back by Ukrainian border troops. Russia's Security Service said that was untrue.

Kiev said Moscow was backing groups in eastern Ukraine who were "putting civilians in danger, seizing hostages and creating an atmosphere of terror and violence".

Reuters journalists in Slaviansk, the most heavily fortified bastion of pro-Russian separatists in eastern Ukraine, heard shooting and saw one helicopter opening fire before dawn. Later in the day, the city was largely quiet, with shops shut and armed separatists in control of the streets.

Advancing Ukrainian forces in armoured vehicles took up positions in the suburbs, but rebels still controlled most of the town of 130,000.

Mr Turchinov said the operation had been complicated by the rebels' use of human shields and had not progressed as quickly as had been hoped.

Chaos casts shadow over looming election

The growing chaos is overshadowing a presidential election the pro-Western leadership in Kiev is planning for May 25. The rebels are planning a vote on May 11 to seek a mandate to break with Kiev, like one held in Crimea before Moscow took it over.

Moscow moved in on Crimea following the overthrow of Ukraine's pro-Moscow president by protesters angered by his decision to scrap a trade deal with Europe.

The United States and Europe have imposed sanctions on individuals over the Ukraine crisis, but they have had limited impact.

US president Barack Obama said the next step would be sanctions on sectors of the Russian economy and they would be imposed if Moscow impeded the Ukrainian presidential poll.

German chancellor Angela Merkel, visiting Washington, told a joint news conference with Mr Obama that the elections on May 25 were critical and sectoral sanctions were ready.

The energy and banking sectors are likely targets.

On the square outside city hall in Slaviansk, about 100 people gathered on Friday and said they were appealing to Mr Putin to send troops to help them.

Businesswoman Tamara Voshchanaya said: "What can you think when the sound of cannon makes you jump out of bed, when helicopters are flying over and shooting at our guys?"

Mr Putin's popularity has soared with the seizure of Crimea and talk of restoring Moscow's former empire.

This week he restored the Soviet-era tradition of holding a May Day parade on Red Square, where marchers carried banners hailing the acquisition of Ukrainian territory.

US defence secretary Chuck Hagel said Russia's actions in Ukraine had shattered the myth of European security in the post-Cold War era and said NATO allies had increased the danger by failing to meet their defence spending pledges.

Helicopters shot down

Meanwhile, the Ukrainian defence ministry said two Mi-24 attack helicopters had been shot down by shoulder-launched missiles while on patrol overnight around Slaviansk. Two airmen were killed and others wounded.

Other Ukrainian officials and the separatist leader in Slaviansk said earlier that one airman was taken prisoner.

A third helicopter, an Mi-8 transport aircraft, was also hit and a serviceman wounded, the defence ministry said.

The SBU security service said this helicopter was carrying medics.

Ukrainian officials said their troops overran rebel checkpoints and Slaviansk was now "tightly encircled".